1. Field of the Invention
The present invention relates to a data collection method and system, relevant center and terminal apparatuses, and a storage medium storing a data collection program and, in particular, to a method and system for efficiently collecting data stored by a large number of unspecified terminals via a public network.
This application is based on Patent Applications Nos. Hei 9-205061 and Hei 9-257238 filed in Japan, the contents of which are incorporated herein by reference.
2. Description of the Related Art
Recently, telecast digitalization is proceeding, and in CS (Communication Satellite) broadcasts, a service on a digital system has already been started. Also in BS (Broadcasting Satellite) broadcasts and broadcasts using ground waves, the start of digitized broadcast services is scheduled. As expected services for such digital broadcasts, a multi-channel broadcast and a two-way interactive communication service have been proposed.
For the conventional national telecast on an analog system in Japan, a data broadcasting method was authorized about two years ago. This broadcasting method itself is similar to the method of the teletext or the caption broadcast, which has been used for 10 years or more. In addition, this data broadcasting method is expected to function as a pilot for the above-mentioned interactive communication service, and each TV station has started their own original service, for example, the "Bitcast" offered by the TBS and Fuji TV stations.
The telephone network is used as a device for collecting data from viewers and supplying it to the broadcasting station in such an interactive communication service. In such a situation, if a great number of viewers are going to send data all together, many problems may occur depending on the amount of the traffic of collected data, for example, congestion of the center window and/or the telephone network. Accordingly, a technique for controlling the data collection is necessary.
Japanese patent application, first publication, No. Hei 8-139806 ("METHOD AND SYSTEM FOR DATA COLLECTION") is an example of conventional data collection controlling techniques. In this system, when a probability value as control information is broadcast (i.e., widely sent) from the center, terminals receive the probability value and the terminals call up the center according to the probability designated by the above probability value.
FIG. 14 is a diagram for explaining a conventional method.
In this figure, when probability value P.sub.0 is broadcast from a broadcasting station, each of many terminals generates a random number and terminals generating a random number which does not exceed the probability value P.sub.0 call up the center. The center counts the number of calls received from the terminals and estimates the number of potentially calling terminals (that is, terminals which desires to call but have not performed yet) based on the above probability value and the counted number of received calls, and then determines and broadcasts a new probability value P.sub.1.
As described above, the center accepts calls from terminals and receives data from the terminals, and simultaneously, counts the number of received calls. The center then estimates the number of all remaining potentially-calling terminals, based on the number of calls after counting and on the already-broadcast probability value. The center determines a probability value to be broadcast next, based on a ratio of the estimated number of potentially calling terminals to the number of possible arrival ports of the center, and broadcasts the new probability value.
The conventional method has a problem in which if a time lag from the broadcast of the control information (from the center) to the arrival of a response call (to the center) is larger than a time from connection to disconnection of a terminal to the center (that is, a service period), then data cannot be efficiently collected. That is, if control information is sent after counting of received calls is completed, then a time period of no call (see shaded areas in FIG. 15) appears before the arrival of the next wave of responsive calls, and thus the center window becomes idle during this time period. Therefore, efficient data collection cannot be performed.